Use of an adhesive sheet for a label having a thermosensitive coloring layer has been increased in recent years as for labels for displaying prices, labels for displaying product information (barcode), labels for displaying quality of products, labels for displaying measurements, labels for advertising (stickers), or the like. As for a recording method thereof, there are inkjet recording, thermosensitive recording, and pressure sensitive recording. Conventionally, a typical adhesive sheet for a label, which has a laminate structure where an adhesive layer and release paper are laminated on an opposite surface of the sheet to a thermosensitive coloring layer, can be used also in the rolled state, as the adhesive layer and the thermosensitive coloring layer are brought into a contact via the release paper. Moreover, the adhesive sheet for a label is widely used because it is easily adhered by peeling the release paper to expose the adhesive layer at the time of adhering.
However, this conventional adhesive sheet for a label requires peeling the release paper upon use. As it is difficult to reuse the release paper as peeled, it is disposed in most cases, which leads to wasting of natural resources.
To solve the aforementioned problem, proposed is a rolled liner-less thermosensitive recording label, which has a release layer formed on a surface of the thermosensitive coloring layer, and having releasing property against the adhesive layer, and thus can used in the state of a roll without the release paper.
This liner-less thermosensitive recording label, however, has the release layer on the surface of the thermosensitive coloring layer, and therefore the release layer is adhered (stacked) on a thermal head as printed by means of a printer equipped with the thermal head, causing a problem that printing cannot be performed adequately. Therefore, there have been needs for a thermosensitive recording label, with which printing can be performed adequately, and in which the adhesive layer and the release layer can be appropriately released from each other.
In order to prevent adhesion between a thermosensitive recording label and a thermal head, and prevent deposition of a release layer forming material onto a thermal head, for example, proposed is heating a release layer forming material (e.g., an emulsion type silicone resin, zinc stearate emulsion, and colloidal silica) with a curing catalyst to sufficiently cure (see PTL 1). In this proposal, however, matching ability of a resulting label to a thermal head is not necessarily adequate. Moreover, as the release layer contains zinc stearate, it is difficult to cure the silicone resin, causing reduction in release ability of the release layer.
Further, proposed is a thermosensitive recording label, in which a thermosensitive coloring layer, a protective layer, and a release layer are laminated, where the protective layer contain a powder having a particle diameter of 0.01 μm to 10 μm to form irregular shapes on a surface of the release layer, to thereby prevent adhesion between a thermal head and the label (see PTL 2). In this proposal, however, a resin for forming the release layer is a UV curing silicone resin, which causes volume shrinkage during UV curing, resulting in low binding ability between the release layer and the protective layer.
Moreover, proposed is to improve curing ability of a release layer formed by curing polysiloxane containing an epoxy group, polysiloxane containing a 1-propenyl ether group, and polysiloxane containing a vinyl ether group by applying radioactive rays, and to improve close contactness between a protective layer and the release layer, using the protective layer containing a water-soluble resin and colloidal silica as main components (see PTL 3). However, this proposal cannot prevent volume shrinkage during UV curing, which leads to a problem that anti sticking property of the release layer is low.
Moreover, proposed is a thermosensitive recording adhesive label containing a release layer formed by curing a material containing a solventless radioactive ray curing organopolysiloxane compound and organohydrogen polysiloxane by application of radioactive rays (see PTL 4). In this proposal, however, the solventless silicone resin has a small molecular weight compared to that of a solvent silicone resin and tends to result insufficient curing bonds. A crosslink reaction proceeds quickly in radioactive ray curing to form a cured coating film within a few seconds. Therefore, ≡SiH groups tend to remain and crosslink density becomes low, which may cause sticking.
Proposed is release paper having a release layer formed by heat curing a solventless silicone resin (see PTL 5). This proposal, however, does not aim to prevent sticking, and to improve binding ability with a barrier layer.
Further, proposed is a thermosensitive recording material, which contains a base, a thermosensitive coloring layer containing a leuco dye and a developing agent, a first protective layer containing a water-soluble resin and a crosslinking agent, and a second protective layer containing a water-soluble resin, a crosslinking agent, and a pigment, where the thermosensitive coloring layer, the first protective layer, and the second protective layer are laminated on the base in this order, and the second protective layer contains diacetone-modified polyvinyl alcohol, and an acrylic resin or maleic copolymer (see PTL 6). In this proposal, however, the protective layer is the outer surface layer, and it does not teach about compatibility with a release layer formed of a solventless silicone resin provided the outermost surface.
As mentioned above, the liner-less thermosensitive recording labels described in the conventional art have problems that a sufficient binding strength between the protective layer and the release layer cannot be obtained, and both anti-sticking property and sufficient release ability of the release layer to the adhesive layer cannot be obtained at the same time.